Pluripotent stem cells have been detected in multiple tissues in the adult mammal, participating in normal replacement and repair, while undergoing self-renewal (Hay, 1966, Regeneration, Holt, Rinehart and Winston, New York; McKay, 1999, Nature Med. 5:261-262; Lemiscka, 1999, Ann. N.Y. Acad. Sci. 872:274-288; Owens and Friedenstein, 1988, Ciba Foundation Syp. 136, Chichester, U.K. pp. 42-60; Prockop, 1997, Science 276:71-74; Ferrari et al., 1998, Science 279:1528-1530; Caplan, 1991, J. Orthop. Res. 9:641-650; Pereira et al., 1995, Proc. Natl. Acad. Sci. USA 92:4857-4861; Kuznetsov et al., 1997, Brit. J. Haemotology 97:561-570; Majumdar et al., 1998, J. Cell Physiol. 176:57-66; Pittenger et al., 1999, Science 284:143-147). A subclass of bone marrow stem cells is one prototype, capable of differentiating into osteogenic, chondrogenic, adipogenic and other mesenchymal lineages in vitro (Owens and Friedenstein, 1988, Ciba Foundation Symp. 136, Chichester, U.K. pp. 42-60; Prockop, 1997, Science 276; 71-74; Ferrari et al., 1998, Science 279:1528-1530; Caplan, 1991, J. Orthop. Res. 9:641-650; Pereira et al., 1995, Proc. Natl. Acad. Sci. USA 92:4857-4861; Kuznetsov et al., 1997, Brit. J. Haemotology 97:561-570; Majumdar et al., 1998, J. Cell. Physiol. 176:57-66; Pittenger et al., 1999, Science 284:143-147). These pluripotent cells have been termed marrow stromal cells (MSCs), and recently have been used clinically to treat osteogenesis imperfecta (Horwitz et al., 1999, Nature Med. 5:309-313).
The recent discovery of stem cell populations in the central nervous system (CNS) has generated intense interest, since the brain has long been regarded as incapable of regeneration (Reynolds and Weiss, 1992, Science 255:1707-1710; Richards et al., 1992, Proc. Natl. Acad. Sci. USA 89:8591-8595; Morshead et al., 1994, Neuron 13:1071-1082). Neural stem cells (NSCs) are capable of undergoing expansion and differentiating into neurons, astrocytes and oligodendrocytes in vitro (Reynolds and Weiss, 1992, Science 255:1707-1710; Johansson et al., 1999, Cell 96:25-34; Gage et al., 1995, Annu. Rev. Neurosci. 18:159-192; Vescovi et al., 1993, Neuron 11:951-966). NSCs back transplanted into the adult rodent brain survive and differentiate into neurons and glia, raising the possibility of therapeutic potential (Lundberg et al., 1997, Exp. Neurol. 145:342-360; Lundberg et al., 1996, Brain Res. 737:295-300; Renfranz et al., 1991, Cell 66:713-729; Flax et al., 1998, Nature Biotech. 16:1033-1039; Gage et al., 1995, Proc. Natl. Acad. Sci. USA 92:11879-11883; Svendsen et al., 1997, Exp. Neurol. 148:135-146). However, the inaccessibility of NSC sources deep in the brain severely limits clinical utility. The recent report demonstrating that NSCs can generate hematopoietic cells in vivo suggests that stem cell populations may be less restricted than previously thought (Bjornson, 1999, Science 283:534-537).
Evidence that MSCs injected into the lateral ventricles of neonatal mice can differentiate to astrocytes and neurofilament-containing cells lends support to this contention (Kopen et al., 1999, Proc. Natl. Acad. Sci. 96:10711-10716).
However, although differentiation of MSCs into astrocytes and glial cells had been demonstrated (WO 99/43286), to date, there has been no method for inducing MSCs to differentiate into neuronal cells. Thus, despite the crucial need for obtaining neuronal cells for treatment of CNS diseases, disorders, and conditions, no method has been available for obtaining large numbers of neuronal cells without encountering the technical and ethical hurdles involved in obtaining human NSCs or fetal tissue. The present invention overcomes this need.